When choosing a WordPress theme in the Appearance sub-menu, the themes usually have an image you can see to identify them and to check them out.  The image cab can be set up by upload a png image to the theme’s parent folder, e.g. for a twentysixteem child them this would be:

/yourdomain.com/wp-content/themes/twentysixteen-child.

The image should be 1200 W X 900 H pixels in size.

The image should be called screenshot.png.

It’s as simple as that…